The present invention relates generally to protective circuits and, more particularly, to overvoltage protection circuits for semiconductor devices such as thyristors which have the capability of being rendered conductive by the simultaneous application of a forward bias voltage and a gating signal applied to a gating electrode of the semiconductor device.
Many semiconductor devices, including thyristors, are susceptible to severe damage if too great a voltage is placed across their anode and cathode terminals. It is known that damage due to such overvoltage can often be prevented if the thyristor is rendered conductive with the attendant reduction in anode to cathode voltage.
It is also known that when the overall voltage to be utilized exceeds the capability of a single device, plural devices may be connected in series; i.e., in a "string". When such series connections are employed, it is necessary that all devices of the string be "fired" (i.e., rendered conductive) simultaneously. One known method of achieving this simultaneous conduction is to employ a plurality of torodial transformer cores (one per device) with the transformer secondaries respectively connected to supply the firing signals to the individual devices. A single turn primary winding is serially passed through each of the cores. Examples of this general technique are found in U.S. Pat. No. 3,654,542 (Apr. 4, 1972) and United Kingdom Pat. No. 1,126,989 (Sept. 11, 1968). Other showings of this concept are also available.
Whether a single semiconductor device or a string of such devices is used, collectively referred to as an array, as indicated above the rendering conductive of the array is an effective means for preventing overvoltage damage to the devices so long as all of the devices of the array are simultaneously rendered conductive. The provision of this protective function, however, presents certain problems since such a protection circuit must be reliable and yet, in a practical commercial sense, economical. The inclusion of such items as separate power supplies and elaborate sensing circuits tend to diminish reliability and increase cost. Additionally, in the event of failure of the normal means of rendering the array conductive, the array devices should be protected against an overvoltage condition.